Letters of Alexander von Humboldt to Varnhagen von Ense. From 1827 to 1858. With extracts from Varnhagen's diaries, and letters of Varnhagen and others to Humboldt
New York, Rudd & Carleton; [etc., etc.], 1860
University of Pittsburgh Library System (archive.org)
Lyrasis Members and Sloan Foundation
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